1. Field of the Invention
The present invention relates to a digital communication system, and more particularly, to TDMA (Time Division Multiple Access) digital wireless communication system.
2. Description of the Prior Art
In digital wireless communication systems, it is often desirable for multiple (i.e. more than two) terminals to communicate with one another, all sharing a single frequency. One method of accomplishing this is commonly known as TDMA, or Time Division Multiple Access, whereby communications occur in a series of consecutive “frames”. A frame is a prescribed block of time, (e.g. 10 milliseconds) during which each terminal is permitted to transmit and receive. All communications over the TDMA system are synchronized to the repetition rate of the series of frames. Each terminal is assigned one or more time-slots within a frame (a time slot is a portion of the duration of the frame) during which it may transmit or receive. Usually, a terminal is assigned at least two time slots: one time slot for transmitting and another time slot for receiving. After the completion of one frame, another frame begins, such that frames occur sequentially in time, and such that during each frame, each terminal has an opportunity to transmit and receive short “burts” of data during its assigned time slots.
An example of a TDMA wireless system is known as the “DECT” (Digital Enhance Cordless Telecommunication) system. In the exemplary DECT system, twelve full duplex communication channels share a single frequency by dividing the transmission time into “Frames”, “Slots”, and “Bits”. FIG. 1A depicts a time-line 100 having a generalized organization of frames and time slots of the DECT system. As a “previous” frame 110A (Frame N−1) ends, a “current” frame 110B (Frame N) commences. As the current frame 110B ends, a “next” frame 110C (Frame N+1) commences, etc. A DECT frame is 10 milliseconds long; it is further divided into 24 equal time slots (slot 0 to slot 23). Each time slot is further divided into 320 Bit-times. Each Bit-time is the time required to transmit one bit of digital information.
A sample protocol 120 for a time slot of FIG. 1A is shown in FIG. 1B, where protocol 120 dictates the information required to be transmitted during a single time slot. Protocol 120 may consist of a header 122, an A-field 124 and a B-field 126. The header 122 is provided for bit and frame synchronization and message-type identification. The A-field 124 is provided for transmitting control signals or less data. The B-field 126 is provided for carrying information during speech or data calls.
FIG. 2 is a schematic block diagram of a prior DECT system 200. When a user sends audio signal via a speaker/microphone 201, an analog-to-digital (A/D) converter 202 converts the audio signal into digitized bit streams. An ADPCM (adaptive differential pulse coded modulation) encoder/decoder 203 encodes the digitized bit streams, then transmitting the encoded data to a burst mode controller 204, and storing in a 1K×8 share RAM (random access memory) 205 of the burst mode controller 204. When the time slot allocated to the user begins, a Frame Packer/De-packer 206 accesses the encoded data stored in the share RAM 205, and packing the data into a packet of DECT specification. Then, the packet is transmitted to a portable as radio-frequency (RF) signal. When the portable transmits audio signal as RF signal to the DECT system 200, the process for transforming the RF signal to audio signal is in reverse.
In FIG. 2, an 8052 micron-controller 207 is used for monitoring the operation of the DECT system 200 and implementing high-level communication protocols and lower-level instructions for controlling the burst mode controller 204. As widespread of mobile phones, e.g. handset, the mobile phones is also used for transmitting message-type data, such as short message, except for telephone calls. In accordance with the block diagram of FIG. 2, only the 8052 micro-controller 207 can be used to transmit the message-type data. However, the micro-controller 207 is busy for implementing high-level communication protocols etc., and its speed is too slow. Thus, the micro-controller 207 cannot take over this task.
Accordingly, it is an intention to develop a message-type data communication apparatus suitable for a DECT system, which can alleviate the above issues and provide bi-directional data transfer with desired transmission rates.